Currently plot_topomap always shows a head centered in the MEG layout, i.e., head centered relative to the sensor positions that have been 3d-to-2d projected device coordinate frame.

We should be able to improve the accuracy of this by translating + "rotating" this head by, according to the information in dev_head_t:

• Show x/y translation by moving the x/y location of the head (in MRG, BUG: Fix M/EEG topomap plotting #7066)
• Show z translation (depth) by shrinking the head (not doing it because it's going to get too confusing with flattening / ortho vs perspective projection)
• EDIT: Not doing this either because it's a bit overkill: Show rotation of the head relative to the sensors by:
  1. Showing a (rotated) cross-hairs on the top of the head (in the center of the head iff the rotation part of dev_head_t is np.eye(3))
  2. Showing the ears and nose rotated properly (in gray if they are not visible because they are rotated downward)

I talked to @agramfort and this should be optional so that people can always get the "standard" plot back. But basically I think how we show things right now is essentially incorrect so I think we should add a parameter transform_head that moves from False(master) ->True (this proposal) via deprecation cycle (users only get a warning when showing MEG data).

This is of course contingent on this representation actually visually looking more informative.